1. Field of the Invention
The present invention relates to an LED array driving apparatus for supplying power to drive an LED array for an LED backlight. More particularly, the present invention relates to an LED array driving apparatus and a backlight driving apparatus using the same which allows regulation of analogue and Pulse Width Modulation (PWM) dimming for each channel and LED of an LED backlight, thereby achieving uniformity of luminance and color in all regions of a backlight.
2. Description of the Related Art
A backlight is a device for illuminating a display panel which used to adopt Cold Cathode Fluorescent Lamp (CCFL) as a light source in the prior art. However, a Light Emitting Diode (LED) has gained popularity recently as a light source since the CCFL was found to have several problems including environmental pollution due to use of mercury, slow response time of about 15 ms, low color reproductibility of 75% compared with National Television System Committee (NTSC), and generation of pre-set white light. Compared with CCFL, the LED is environmentally friendly, is possible in high-speed response in nano-seconds, is possible in impulse driving, is 100% in color reproductibility, and is possible in regulation of luminance and color temperature of a backlight by adjusting luminous flux of red, blue, and green LEDs.
In the prior art, the LED driving circuit used as a light source of a backlight may take a form of buck or boost type DC-DC converter to turn on or off the LED.
FIG. 1 illustrates an LED array driving circuit for a buck type backlight proposed in the prior art in which a DC-DC converter 11, which raises supply voltage to a predetermined DC level, is connected to an anode of the LED array 10 having a grounded cathode. The DC-DC converter 11 includes a transistor Q1 disposed in series on the power line to switch on or off; a PIN diode D1 connected in reverse direction between an output end of the transistor Q1 and a ground; an inductor L1 for connecting the output end of the transistor Q1 with the LED array 10; and a capacitor C1 disposed between the contact point of the inductor L1 with the LED array 10 and the ground.
In addition, the LED array 10 is driven by constant voltage with an error amplifier 12 for using an output voltage applied from the DC-DC converter 11 to the LED array 10 as a reference voltage of a predetermined level, a comparator 14 for comparing an output signal of the error amplifier 12 with a signal applied from a local oscillator 13, and an operation amplifier 16 for current-limiting the output signal from the comparator 14 to apply to the transistor Q1 as a switching regulation signal. In the above process, the current limiter 15, connected to the operation amplifier 16, has the function of regulating the current-limiting operations.
However, when the above prior art driving circuit is used to drive the LED array with a plurality of LEDs connected in series, luminous flux varies for each LED due to deviation in forward voltage of each LED. Thus, in order to reduce the deviation in luminous flux between the LEDs connected in series, constant-current driving rather than constant-voltage driving is required.
If the backlight with the above described driving circuit is a vertical-descent type with an LED located in the lower part of the display panel, since a plurality of LED arrays are disposed in a predetermined interval to one another, and each LED array has a driving circuit of FIG. 1, independent driving may cause deviation in luminous flux for each LED array. In addition, in case of a backlight using a side illumination type LED, there occurs a phenomenon of luminance at the center being higher than the peripheral part due to the optical and mechanistic properties of a backlight unit, which requires regulation of luminous flux for each location.
In other words, a prior art driving circuit cannot satisfy the above described needs and particularly, it has not succeeded in taking advantage of the merit of LED which is being able to change luminance and color temperature.